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University of Groningen In-situ element analysis from gamma-ray and neutron spectra using a pulsed-neutron source Maleka, Peane Peter IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2010 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Maleka, P. P. (2010). In-situ element analysis from gamma-ray and neutron spectra using a pulsed-neutron source Groningen: s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 16-06-2017 CHAPTER 2 INTRODUCTION In 1803, Dalton proposed a set of postulates to describe the atom. In his model, all matter was made of small particles called atoms. Also atom was considered an elementary particles i.e. it was impossible to divide it into smaller particles. All of the results of chemical experiments during that time indicated that the atom was indivisible. With Thomson's discovery of the electron in 1897, it had already become clear that atoms must have structure. Electrons are negatively charged particles and constitute about 0.05% of the hydrogen atom mass. The notion of electron mass led to the assumptions that most of the mass of an atom must reside in its positively charged particle. Rutherford postulated in 1911 the atomic model in which the positive charge and most of the mass of the atom resides in a very small region, less than 10-12 cm in diameter and a small number of electrons enough to balance the positive charge are distributed over a sphere of atomic dimensions. This positive centre later became known as the nucleus. In follow-up studies, Rutherford discovered the proton in 1919. Protons are relatively large particles that have almost the same mass as a hydrogen atom and a positive charge equal in magnitude to that of an electron. Experiments by Rutherford revealed that the nuclear mass of most atoms surpassed the number of protons it possessed; this led him to postulate the neutron whose existence would only be proven in 1932 by Chadwick. This third subatomic particle, neutron, is slightly heavier than the proton and is electrically neutral. In 1895 Röntgen already discovered that when cathode rays struck certain materials a new type of radiation was emitted, which he called X-rays. Becquerel in 1896, while studying the effect of X-rays on photographic film, discovered that some materials spontaneously decompose and give off very penetrating rays. Pierre and Marie Curie in 1898 studied uranium and thorium and called their spontaneous decay process radioactivity. They also discovered the radioactive elements polonium and radium. Gamma-rays were discovered in 1900 by Villard. While studying uranium he discovered that the rays were not bent by a magnetic field. At that time, gamma-rays were assumed to be particles. In 1914, Rutherford and Andrade showed that gammarays were a form of electromagnetic radiation by measuring their wavelengths using crystal diffraction. The wavelengths are similar to those of X-rays and are very short, in the range 10-11 to 10-14 m. The distinction between the X-ray and gamma-ray depends on the source of the radiation, X-ray photons are emitted in transition of electrons between atomic shells and gamma-rays are emitted by excited nuclei in their transition to lower-lying nuclear levels. 2.1 Neutron properties Neutrons are the uncharged constituents of the nucleus and hence are highly penetrating (see subsection 2.3.1). Neutrons are hadrons that are composed of two 1 down and one up quark, have spin-parity ( ) and a magnetic moment. The neutron 2 (at thermal energies) wavelengths are similar to atomic spacings. Outside the nucleus, neutrons are unstable and called free neutrons with an average lifetime of (886.7 13 INTRODUCTION 1.9) s. Free neutrons decay (beta (-)) by emitting an electron (e-) and antineutrino e to become a proton (p); n p e e (2.1) Neutrons are mainly classified according to their energies and their classifications are presented in table 2.1. One other group of neutron class not specified in table 2.1 is called the slow neutron group, referring to neutrons with energies less than 1 eV, i.e. from epithermal neutrons down to cold neutrons. Table 2.1: Classification of neutron energies. Energy En < 0.025 eV En 0.025 eV 0.025 eV < En < 1 eV 1 eV < En < 500 keV En > 0.5 MeV 2.2 Class Cold neutrons Thermal neutrons Epithermal neutrons Intermediate region Fast neutrons Gamma-ray properties A gamma-ray is a packet of electromagnetic energy emitted by the nucleus of some unstable, radioactive atoms. Gamma-rays have no mass and no electric charge. Gamma-rays are emitted in the form of photons, discrete bundles of energy that have both wave and particle properties. Gamma-ray photons have the highest energy in the electromagnetic radiation spectrum and consequently their waves have the shortest wavelength. These photons carry the energy released in a transition between states in a nucleus. The time characteristics of their emission represents the half-life time of the initial nuclear state, which is commonly a fraction of a second. Often gamma-ray emission occurs after β-decay. The time characteristics then reflect the lifetime of the β-decaying states and this may be very long, up to years. Gamma radiation is a very high-energy ionising radiation, implying that it has enough energy to remove tightly bound electrons from atoms. 2.3 Interaction of radiation with matter Various types of radiation interact with matter in several ways. A large, massive, charged alpha particle, emitted in natural decay, does not penetrate a thin sheet of paper and even has a limited range in air. A neutrino, at the other extreme, has a low probability of interacting with matter and can pass through the diameter of the Earth or the Sun without being absorbed. 14 2.3.1 Neutron interaction with matter Since neutrons are uncharged, their interaction with electrons in matter proceeds via the magnetic moments of the two particles rather than the Coulomb force. The neutrons interact with the nuclei of atoms through the strong nuclear interaction. This hadronic interaction has a very short ranged, which means that the neutrons have to pass close to a nucleus for an interaction to occur. Because of the small size of the nucleus in relation to the atom, neutrons have a low probability of interaction and can therefore travel considerable distances in matter. When a neutron interacts with an atomic nucleus, the neutron can be scattered (deflected or slowed down) or captured (absorbed). Elastic and inelastic scattering of the neutron means that the atomic nucleus remains unexcited (in ground state) or is excited (into an excited state), respectively. If a neutron is absorbed by a nucleus, a compound nucleus is formed. The compound nucleus usually is formed in an excited state and will decay by combination of gamma-radiation, particle emission or for heavy nuclei by fission. A neutron-scattering reaction occurs when a nucleus, after having been struck by a neutron, emits a single neutron. Despite the fact that the initial and final neutrons do not need to be the same, the net effect of the reaction is as if the projectile neutron had merely "bounced off" or scattered from the nucleus. In an elastic scattering process shown in figure 2.1 between an incident neutron and a target nucleus, there is no net energy transferred into nuclear excitation. Elastic scattering of neutrons by nuclei can occur in two ways. The more unusual of the two interactions is the absorption of the neutron, forming a compound nucleus, followed by the re-emission of a neutron in such a way that the total kinetic energy is conserved and the nucleus returns to its ground state. This is known as resonance elastic scattering and depends upon the initial kinetic energy of the neutron, the atomic mass (A) and atomic number (Z) of the nucleus. Due to the formation of the compound nucleus, it is also referred to as compound elastic scattering. The other more usual method is the potential elastic scattering and this can be understood by visualizing the neutrons and nuclei to be much like billiard balls with impenetrable surfaces. Potential scattering takes place with incident neutrons that have energy of up to about 1 MeV. In potential scattering, the neutron does not actually touch the nucleus and no compound nucleus is formed. Instead, the neutron is acted on and scattered by the short-range nuclear force when it approaches close enough to the nucleus. The two interactions are not distinguishable experimentally. Figure 2.1: Schematic presentation of the neutron elastic scattering process (CANDU04). 15 INTRODUCTION Momentum and kinetic energy of the system are conserved. The target nucleus gains the amount of kinetic energy that the neutron loses and the scattering angles are determined by the conservation of momentum. The maximum energy Qmax that a neutron of mass m and kinetic energy En can transfer to a target nucleus of mass M in a single elastic collision occurs in a head-on collision (Krane, 1988); Qmax 4 m M En (2.2) m M 2 4 m En for M >> m M (2.3) Expressing masses in units of atomic mass numbers, the neutron has mass m = 1 and the maximum fraction of a neutron's energy that can be lost in a collision with nuclei of atomic-mass number (M) rapidly decrease for M ≠ 1. In a collision with hydrogen nucleus (atomic mass of 1, the same mass as the neutron), the neutron can loose almost all of its energy according to eq. 2.2 (Knoll, 2000). From eqs. 2.2 & 2.3, it follows that elastic scattering on a light nucleus is the most efficient way to moderate (slow down) fast neutrons. In an inelastic scattering process, schematically shown in figure 2.2, the incident neutron enters the nucleus for a brief period and forms a compound nucleus. The compound nucleus will then emit a neutron (any) and a gamma-ray photon, and thus will be reverting back to the target nucleus. The direction of the emitted neutron is more/less random. A distinction between the elastic and inelastic scattering is that elastic scattering can occur at any neutron energies while the inelastic process requires excitation of the nucleus and is most probable for fast neutrons (via threshold reactions). Figure 2.2: Schematic presentation of the neutron inelastic scattering process (CANDU04). Instead of re-emitting a neutron as in inelastic scattering, the compound nucleus may emit an alpha particle or a proton. This process is called particleemission reaction and is illustrated for an alpha particle (4He) emission in eq. 2.4. 1 n 10 B B 11 * 16 7 Li 4 He (2.4) In a radiative capture reaction, the compound nucleus looses its excitation energy by emitting a gamma-ray. In contrast to the inelastic scattering, the neutron is not re-emitted, the nucleus is converted into a heavier isotope of the target-nucleus element. An example of a radioactive capture reaction is shown in eq. 2.5. Radiative capture can practically occur with all types of nuclei and at all neutron energies, but is more probable with slow neutrons than with fast neutrons. 1 n 1 H 2H (2.5) Another possible reaction is fission, in which a heavy nucleus that absorbs the neutron splits into two or more medium-heavy fragments. Table 2.2 summarises the neutron interactions with atomic nuclei classified according to their energy. The classifications in table 2.2 illustrate the more probable interactions with atomic nuclei and the neutron energies are represented by three classes from table 2.1. Table 2.2: Classification of neutron interactions with atomic nuclei (Beckurts and Wirtz, 1964). Fission is denoted by f. Slow neutrons (En < 1 keV) Intermediate energy (1 keV < En < 500 keV) Fast neutrons (0.5 MeV < En < 20 MeV) Potential (elastic) scattering Light nuclei (A < 25) Resonance scattering; Reactions (n,p), (n,), (n,2n) Resonance scattering; Radiative capture Intermediate nuclei (25 < A < 80) Potential scattering Inelastic scattering; Reactions (n,p), (n,),.. (n,2n) Inelastic scattering Heavy nuclei (A > 80) Radiative capture Reactions (n,f), (n,2n) 17 INTRODUCTION 2.4 Neutron interaction cross section The rate of neutron interactions depends on the incoming neutron flux and the cross section for interaction. The cross section is the probability of a neutron interacting with the material. The cross section for a particular reaction does not only depend on the kind of nucleus involved but also on the energy of the neutron. The absorption of a slow neutron in most materials is much more probable than the absorption of a fast neutron. At low energies, neutron cross sections are smooth functions of energy while at higher energies they are dominated by resonances due to the presence of favourable states in the compound nucleus at these energies, see for example figure 2.3. An illustration in figure 2.3 shows that in the inelastic scattering with an 56Fe nucleus a neutron energy of at least 1 MeV is required. Both the capture reaction and elastic scattering are probable at all neutron energies with resonance structures clearly dominating at higher energies. 3 Neutron cross-section (barns) 10 Capture Elastic scattering Inelastic scattering 2 10 1 10 0 10 -1 10 -2 10 -3 10 -4 10 -11 10 -9 10 -7 -5 10 10 -3 10 -1 10 1 10 En (MeV) Figure 2.3: An example showing the cross section for three neutron interaction processes as a function of energy with the nucleus, 56Fe (Data source - JEF-PC Software, 1998). The possibility of a particular reaction occurring between a neutron and a nucleus can be derived from the microscopic cross section (σ). The microscopic cross section may also be regarded as the effective area that the nucleus presents to the neutron for a particular reaction. The larger the effective area, the greater is the probability for the reaction. The microscopic cross section is an area and is often expressed in units of barn, and 1 barn is equivalent to 10-28 m2. Microscopic neutron cross-section data are determined from experimental data as a function of energy for each nuclide and each reaction. In general, these data cannot be interpolated over large energy intervals because of the irregular resonance structures. Shown in figure 2.4 is an example of the total (i.e. due to elastic, inelastic, capture, etc) microscopic cross section plots for the 56Fe and 57Fe nuclei as a function 18 of energy. The differences between two iron isotopes 56Fe and 57Fe indicate that the data cannot be interpolated readily between nuclides; each nucleus has its own structure. As a result, measurements, calculations and evaluations of neutron cross sections for a particular element have to be separately worked out for each isotope, tabulated and updated whenever new information becomes available. 3 10 Neutron cross-section (barns) 56 Fe 57 Fe 2 10 1 10 0 10 -1 10 -11 -9 10 10 -7 -5 10 10 -3 10 -1 10 1 10 En (MeV) Figure 2.4: Total microscopic cross section for two iron isotopes, 56Fe and 57Fe (Data source - JEF-PC Software, 1998). Whether a neutron will interact within a certain volume of material depends not only on the microscopic cross section of the individual nuclei but also on the number of nuclei within that volume. Therefore, it is suitable to describe another type of cross section known as the macroscopic cross section (Σ). The macroscopic cross section is the probability of a given reaction occurring per unit length the neutron travelled. The macroscopic cross section, Σ (m-1) is related to the microscopic cross section σ (m2) by the relationship shown in eq. 2.6 and is a function of neutron energy E. E N E (2.6) with N (m-3) the atom density of the material defined in eq. 2.7 below. The atom density is the number of atoms of a given type per volume of the material. The atom density for individual materials can be calculated using the equation, N NA (2.7) M 19 INTRODUCTION where is the density of the material in (kg m-3), NA the Avogrado number (6.022 x 1023 mol-1) and M the atomic weight in (kg mol-1). Macroscopic cross sections have the dimension of inverse length, and are interpreted as the probability per unit path length that a specific process/interaction will occur. Microscopic cross sections represent an effective target area that a single nucleus presents to a bombarding particle, while a macroscopic cross section represents the effective target area presented by all nuclei contained in 1 m 3 of material. The average distance travelled by a neutron before interaction, known as the mean free path (m), is related to the macroscopic cross section Σ (m-1), by 1 (2.8) Macroscopic cross sections for neutron reactions with materials determine the probability of one neutron undergoing a specific reaction per metre of travel through that material. The number of actually occurring reactions will depend on the neutron flux. The distance these neutrons can travel per second will be determined by their mean velocity, v (m s-1). A way of defining the neutron flux () is to consider it to be the total path length covered by all neutrons in one cubic metre during one second. Mathematically, this translate to eq. 2.9, nv (2.9) where (m-2 s-1) is the neutron flux, n (m-3) the neutron density and v (m s-1) the neutron velocity. The neutron density, n (m-3), is the number of free neutrons moving through a unit volume of material. The rate Rx (m-3 s-1) at which a particular nuclear reaction will take place is related to the neutron f l ux, the cross section for the interaction and the atom density of the target as, Rx ( N ) (2.10) (2.11) where (m-2 s-1) is the neutron flux, N (m-3) the atom density, (m2) the microscopic cross section and (m-1) the macroscopic cross section. It should be noted that eqs. 2.10 and 2.11 hold as well for a particular energy (see also eq. 2.6) as for an interaction over a certain energy range. The intensity of the neutron beam will decrease exponentially when traversing a material with thickness d by (Lewis and Miller, 1984), I I 0 e tot d (2.12) where, tot is the total macroscopic cross section for all processes occurring in the material: tot inelasticscattering elasticscattering capture.... 20 (2.13) Equations 2.12 and 2.13 hold for mono-energetic beams as well as for beams with a particular energy distribution. 2.5 Gamma-ray interaction with matter Because gamma-rays have no mass and no charge, they need, like neutrons, an indirect way to transfer energy. Therefore gamma-rays, compared to charged particles have high penetrating power. Gamma radiation interacts with matter through electrons mainly via three processes: photoelectric effect, Compton scattering and pair production. 2.5.1 Photoelectric effect In the process of photoelectric absorption the incident gamma-ray photon is completely absorbed by an electron of an atom. The photoelectron is ejected with a kinetic energy: Ee E Eb (2.14) with E the photon energy and Eb the binding energy of the electron in its original shell. Usually this binding energy is small compared to the photon energy and can be ignored. There is no analytical expression describing the cross section for the entire range of gamma-ray energies (E) and atomic numbers Z. The general trend for the cross section per unit mass for the photoelectric absorption (m2 kg-1), can be approximated by: Const. Z n E3.5 (2.15) with n varying between 4 to 5 over the gamma-ray energy region of interest (Knoll, 2000; Koomans, 2000). The strong Z dependence indicates that a high-Z material is very effective in the absorption of low-energy -ray photons. 2.5.2 Compton Scattering In the process of Compton scattering, a -ray photon is scattered off an electron of an atom and energy and momentum are transferred to the recoil electron according to the conservation laws of momentum and energy. The effect is schematically illustrated in figure 2.5. The photon loses only part of its energy to the recoil electron. From the conservation of energy and momentum, the energy E ' of the outgoing photon is related to the scattering angle at which it is emitted by: 21 INTRODUCTION E ' E E 1 2 1 cos m0c (2.16) with E the incident photon energy, m0c2 = 0.511 MeV, the rest-mass energy of an electron. The recoil energy of the electron Ee is therefore given by: Ee E E ' (2.17) E' E Ee Figure 2.5: Schematic presentation of a Compton-scattering process (Hendriks, 2003). The angular distribution of scattered gammaNishina formula for the atomic differential scattering cross section d/d: d Zr02 1 d 2 1 1 cos 2 2 1 cos 2 1 cos 2 1 1 cos - (2.18) where E /m0c2 (photon energy in units of electron rest energy), r0 is the classical electron radius and Z the charge of the scattering nucleus (Knoll, 2000). This distribution becomes more and more forward peaked with increasing photon energy. The probability of Compton scattering decreases with increasing photon energy. Compton scattering is considered to be the principal absorption mechanism for gamma-rays in the intermediate energy range: 100 keV to 10 MeV. The cross section for Compton scattering per unit mass (m2 kg-1), is given by: Const. E1 (2.19) It follows from eq. 2.19, that the probability for Compton scattering is independent of the atomic number of the material through which the gamma-ray traverses, and only depends on its density. 2.5.3 Pair-production In pair production, the photon is converted into an electron-positron pair in the Coulomb field of the nucleus. Since the energy of twice the rest mass of the electron 22 (m0c2) is required to create the electron-positron pair, this process only occurs for E > 1.022 MeV. The excess energy equal to the photon energy minus 1.022 MeV (2 x m0c2), is shared by the electron and positron as kinetic energy. When the positron has lost its kinetic energy it will form an intermediate state with an electron (positronium) and subsequently annihilate with an electron, producing two photons with energy E = 0.511 MeV. To conserve momentum, the two photons are emitted in opposite directions in the centre of mass system of the electron-positron pair. Since the process mainly occurs after the positron has slowed down also in the laboratory system, the two photons are emitted back to back. The cross section for pair-production per unit mass (m2 kg-1) increases with atomic number Z and the square of the gamma-ray energy E: Const. Z E2 (2.20) Pair-production becomes the dominant process for photon energies E > 3 MeV. 2.6 Implications for neutron detection Because neutrons produce no direct ionisation events, neutron detectors are based on detecting the secondary events produced by nuclear reactions, such as (n,p), (n,α), (n,γ) or by nuclear scattering from light charged particles which are then detected (Lilley, 2001; Knoll, 2000; Krane, 1988). The general principle of detecting neutrons involves a two-step process; first the neutron must interact in the detector to form charged particles and secondly the detector must then produce an output signal based on the energy deposited by these charged particles. Thus the key aspects to effective neutron detection are hardware and software. Hardware refers to the kind of neutron detector used (the most common today is the scintillation detector) and to the electronics used in the detection set-up. Software consists of analysis tools that give for example graphical results for analysing the number and energies of secondary particles reaching the detector. The most widely used methods for fast neutron detection is based on elastic scattering of neutrons by light nuclei (Knoll, 2000). In this interaction, an incident neutron transfers a portion of its kinetic energy to the scattering nucleus, resulting in a recoil nucleus and a neutron, see also subsection 2.3.1 and figure 2.1. Because the target materials are mostly light nuclei, the recoil nucleus behaves much like a proton or alpha particle as it loses its energy in the detector medium. The most effective target nucleus is hydrogen because its cross section for neutron elastic scattering is relatively large and its energy dependence is accurately known. Neutrons can transfer an appreciable amount of energy in one collision with a hydrogen nucleus or even lose all their energy. The energy (ER) given to the recoil nucleus is uniquely determined by the scattering angle θ (see also eq. 2.2). ER 4A 1 A2 cos 2 23 En (2.21) INTRODUCTION where A is the mass number of the target nucleus, En the incoming neutron energy. A head-on collision of the incoming neutron with the target nucleus will lead to a recoil in the same direction ( 0), thus resulting in the maximum possible recoil energy. For such collisions, eqs. 2.2 and 2.21 show that in a single collision with a hydrogen nucleus (A =1), a neutron can transfer all its energy whereas only a small fraction can be transferred in collisions with heavy nuclei. When a mono-energetic fast neutron strikes a material containing hydrogen, the energy spectrum of the recoiling protons forms a continuum extending from zero (for small angle scattering in eq. 2.21) to full energy for head-on collisions. Some information about the original energy of the neutrons can be deduced by recording the pulse-height spectrum from a hydrogen-containing detector. Thus for a hydrogenous material bombarded with neutrons of energy En, the spectrum W(Ep) of recoil protons in the laboratory system will have a rectangular shape (Lilley, 2001; Krane, 1988) of the form: W ( E p ) const (2.22) W (E p ) 0 (2.23) for Ep En and for Ep > En. Because of non-linear light-response of the scintillator detectors, amplitude resolution, edge effects and multiple neutron scatterings by the detector materials, the measured spectrum deviates from the almost perfect rectangle (Krane, 1988), illustrated also in figure 2.6. Similarly to eq. 2.21, for momentum and energy conversation in an elastic collision, the final energy of the neutron (Ef) after scattering is given by, E f En ER (2.24) A 2 1 2 A cos E f En A 12 (2.25) or where En is the incoming neutron energy, A the mass number of the target nucleus and the scattering angle (Krane, 1988). The average number of collisions, nc, which is required to moderate a fast neutron with kinetic energy En to kinetic energy Ef in hydrogen (i.e. A = 1) is equal to: E nc ln n E f 1 ln En E f (2.26) E with the parameter ln n known as the logarithmic energy decrement, ξ. E f av For A > 1, 24 1 A 12 ln 2A A 1 A 1 (2.27) The average value of lnEf decreases after each collision by , and after n collisions, the average value of lnEf is given by eq. 2.28; ln E f ln En nc (2.28) Ep = En Ep Figure 2.6: Schematic presentation of an ideal spectrum of proton recoils induced by mono-energetic neutrons and the modification of such a spectrum caused by detector resolution and scintillator non-linearity (Krane, 1988:p455). The cross section for neutron interactions in most materials is a strongly varying function of neutron energy, hence several methods are developed for neutron detection in various energy regions, see table 2.1 and table 2.2 for example. For slowneutron detection, neutrons are detected via nuclear conversion reactions to a charged particle (for example (n,α), (n,p)) (Knoll 2000; Krane, 1988) and all techniques used to detect slow neutrons are based on charged particle detection (Knoll, 2000). For nuclear reactions that will be useful for slow neutron detection, it is important that the cross section for the reaction is as large as possible so that efficient detectors can be built with small dimensions. Hence most of the widely used slow-neutron detectors contain (10B) or (6Li). The thermal neutron cross sections for the 10B(n,α)7Li and 6 Li(n,α)3H reactions are 3840 and 940 barns, respectively (Knoll, 2000). An example given by Knoll (2000) estimates that with a 10 mm thick crystal prepared with highly enriched 6LiI, the efficiency is almost 100% for neutrons with energies from thermal (0.025 eV) to the cadmium absorption energy of 0.5 eV (Knoll, 2000). Thus 6Li is one of the favourite components of a crystal because of its relatively large capture cross section for thermal neutrons. The reaction shown in eq. 2.29 illustrates the process, 6 Li 1 n 4 He 3 H 25 (2.29) INTRODUCTION where the Q-value of about 4.78 MeV is shared between the alpha particle ( He) and the tritium (3H) (Knoll, 2000). 4 2.7 Implications for gamma-ray detection Radiation detectors will in principle give rise to an output signal for each quantum of radiation that interacts within its active volume. For the detector response, gamma-rays must undergo one of the interaction mechanisms in the detector as discussed in section 2.5. When a gamma-ray enters a detector medium, it transfers part or all of its energy to an atomic electron, thus freeing the electron from its atomic bond. This freed electron usually transfers its kinetic energy in a series of collision to other atomic electrons in the detector medium as illustrated in figure 2.7. For a gamma-ray detector, it must act as a conversion medium in which incident gamma rays have a reasonable chance of interacting to yield one or more fast electrons and also function as a conventional detector for these secondary electrons. The top part of figure 2.7 shows a typical example of the interaction processes that a photon will likely undergo inside the detector crystal, and the bottom part shows an example of a typical resulting gamma-ray spectrum. As shown in figure 2.7, the dimensions of the detector medium are also important, especially for Compton scattering processes occurring at the edges of the detector medium. The electron range with kinetic energy of a few MeV in typical solid detector media is a few millimetres (Knoll, 2000). For the photoelectric absorption process, as long as it happens within the detector medium, the information about the gamma-ray can be derived. The bottom part of figure 2.7 shows a spectrum of the energy deposited in the detector by incident photons. Its main feature is the full-energy peak (often called photopeak). This peak stems from those events in which, in one or more steps, the full photon energy was deposited in the detector. The peak position corresponds to the energy of the original gamma-radiation, E = h. This makes the full-energy peak a very useful feature in gamma-ray spectrometry. The continuum results mainly from the single or multiple Compton-scattering events. In a Compton-scattering process, a variable part of the photon energy is deposited via electrons, followed by the escape of the scattered photon. This continuum reaches up to the Compton edge, which corresponds to the maximum electron energy given by eq. 2.16. A succession of multiple Compton events can result in a total deposited energy, which is higher than the value corresponding to the Compton edge. For gamma-radiation with E > 1.022 MeV pair production can add two more peaks to the spectrum. The single escape peak correspond to the initial pair production interactions in which only one annihilation photon leaves the detector without interaction and the double escape peak is due to events in which both annihilation photons escape. 2.8 Summary and conclusion In this chapter, the interactions of neutral particles like neutrons and gammarays with matter have been described. An understanding of all these processes discussed will be important to unfold and analyse the signal spectra obtained by the NuPulse instrument radiation detectors. Neutrons are mainly detected via recoil protons and charged particles produced in nuclear reactions, whereas gamma-rays are 26 detected by their interaction with electrons. The processes form the basis for the detectors used in the NuPulse instrument design and optimisations e.g. via Monte Carlo simulations. In the analyses and discussions of the spectra obtained with the NuPulse instrument the processes described in this chapter will be used. The next chapter introduces the NuPulse instrumentation, optimisations and analysis procedures. A description of all components is mapped out including some possible tool configuration. Chapter 3 will also introduce the reader to some of the test facilities where the assembled tool was tested and optimised at the KVI. Figure 2.7: Schematic example of γ-ray interaction processes in a detector medium and the resulting spectrum for a mono-energetic source (from Knoll, 2000: p316). 27 INTRODUCTION 28